As an example of a backup method of data, there is a method of duplicating data in memory devices controlled by storage control devices. In the method, when a first storage control device receives a request to write the data from a host device, the first storage control device writes the data in its own memory and also transmits the data to a second storage control device so that the data is to be written in a memory controlled by the second storage control device thereby duplicating the data. With this backup method, for example, it is possible to reduce the possibility of data loss due to, for example, an occurrence of a disaster by providing the respective storage control devices at physically separate locations from each other. In the backup method above, an operation mode, in which the first storage control device that has received the request of data writing from the host device transmits a writing completion response to the host device after completion of data duplication, is referred to as, for example, a “synchronous mode”.
As an example of a data backup technology, there has been suggested a technology in which write data that has been received by a first memory subsystem from a host terminal is received by a second memory subsystem. In this technology, the first memory subsystem generates a data set which includes the write data and an update number indicating an update order of a memory device included in the first memory subsystem. The first memory subsystem transmits, to the second memory subsystem, write data corresponding to an update number designated by a read command received from the second memory subsystem.
As another example, there has been suggested a technology in which a first memory system transmits, to a third memory system, write data and address information indicating a storage location where the write data is written, and a second memory system receives the write data from the third memory system according to the address information.
As an example of a data write request sequence, there is known a sequence in which, when a host computer transmits a write request, a storage system returns a completion notification of data receiving preparation, and the host computer that has received the completion notification transmits write data.
Related techniques are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2006-65624, Japanese Laid-Open Patent Publication No. 2005-55948, and Japanese Laid-Open Patent Publication No. 2007-219717.
In the backup process in the synchronous mode, when a first storage control device that has received a write request of data from a host device transmits write data to a second storage control device, the write request sequence above may be used. In this case, the first storage control device transmits a write request to the second storage control device, receives the completion notification of receiving preparation from the second storage control device, and transmits the write data to the second storage control device.
In this case, the first storage control device needs to perform a communication in one round trip with the second storage control device before transmitting the write data, and there is a problem in that a time for responding to the host device is prolonged to that extent. In particular, as the physical distance between the storage control devices becomes longer, the time required for the communication between the storage control devices is prolonged. As a result, the time for responding to the host device is further prolonged.
Besides the backup process in a synchronous mode, the write request sequence above may be used in a write request in a case where the first storage control device that has received write data from the host device transmits the write data to the second storage control device so that the write data is written in a memory device of the second storage control device. A similar problem may occur in this case as well.